In an optical record disc of the type presently used with a DAD (Digital Audio Disc) player or a video disc player, audio or video information is recorded in optically coded form as a series of recesses or pits formed in the information carrying face of the optical record disc and arranged in either a single spiral track or a plurality of concentric tracks about the center axis of the optical record disc. The audio or video information thus recorded is reproduced by optically scanning the individual recesses or pits along the spiral track or the concentric tracks by means of a convergent spot of laser light beam developed from a laser light beam which is utilized to detect the lengths and spacings between the scanned recesses or pits. During reproduction, the optical record disc is usually rotated about the center axis thereof at a constant linear verocity (CLV) or a constant angular verocity (CAV), and the convergent spot of the laser light beam is displaced radially relative to the optical record disc by a tracking device or a pick-up unit which is a part of the DAD or video disc player. The laser beam is directed to a selected track location and reflected by the information carrying face of the otical record disc or is passed through the optical record disc. The beam of light thus reflected by or passed through the optical-disc is then converted into an electric signal by means of a photoelectric transducer mounted on the pick-up unit so as to facilitate further conversion into an audio or video signal.
In order to reproduce the information recorded on the optical record disc, the convergent spot of the laser light beam must be precisely applied to the selected track location on the optical record disc. To this end, it is known to employ a method in optical record disc information reproducing apparatus wherein light beams, exclusively used for scanning, are generated separately from the light beam used for reproducing the audio or video information signal, with a photoelectric transducer being used for detecting the scanning light beam and effect scanning control. Another method employed involves extracting the audio or video information signal and the scanning signal all with a single light beam.
According to the methods, the light beam for the tracking servo is reflected by the optical record disc, then introduced into a photoelectric transducer. The light beam thus reflected is converted into a tracking error information signal. The tracking error information signal thus detected is introduced into a tracking actuator for driving an objective lens mounted on the pick-up unit radial relative the optical record disc so as to direct the convergent spot to the selected track location accurately. A tracking actuator comprises a tracking servo for the convergent spot of the laser light beam.
Generally, the tracking servo must have relatively high response characteristics to make the convergent spot strictly follow quick fluctuations of the selected track location typical during reproduction conditions. However, when dropouts occur due to defects like dust or scars on a surface of the optical record disc, the tracking actuator mistakingly causes the convergent spot to jump from the correct track location. Because the tracking error information signal becomes large due to the defects so that the tracking actuator drives the objective lens mistakingly in response to the tracking error information signal. Conventional tracking servo systems therefore been designed which lower their response characteristics upon detection of information dropout.
Tracking systems employed to date have several drawbacks, however. The convergent spot is still apt to jump from the selected track location because the tracking actuator fail to vary their response characteristics when defects like dust or scars are of those left inside of the optical record disc during its manufacturing process. Here, as well-known, the optical-disc comprises a transparent disc which bears on its one surface the series of the recesses or the pits and a reflecting film coated on the surface of the transparent disc. The defects are apt to be left inside of the optical record disc, that is, on the recesses or pits bearing surface of the transparent disc during its manufacturing process. The defects outside the surface of the the optical record disc not only cause the tracking error information signal to be increased but also cause the reproduction signal to be reduced to an insufficient level. While the defects inside of the optical record disc cause the tracking error information signal to be increased but cause the reproduction signal to be remained in a relatively sufficient level. Influences on the reproduction signal and the tracking error information signal caused by the defects outside or inside of the optical record disc will be explained in detail after description of preferred embodiments of the present invention.
Therefore, the conventional tracking servo system fails to prevent a malfunction of a track jump caused by the defects left inside of the optical record disc. Because the the information dropout necessary for the control of lowering the response characteristics fails to be detected for the defects left inside of the optical record disc.
It should be apparent to those knowledgeable in the field of scanning beam control systems, and particularly the field of scanning beam tracking systems, that there is a definite need for a system that can detect and compensate for deviations from normal operation of the specified characteristic being controlled. In particular, it should be apparent from the forgoing that a specific need has existed for a tracking system that includes means for detecting and compensating for record medium defects that otherwise can cause a loss of tracking. The present invention fulfills these needs.